Among organic and inorganic solid materials containing relatively large amounts of mechanical and/or chemically combined water are those that may be considered temperature sensitive in that they have relatively low melting temperatures. As a consequence drying these low melting materials presents many problems. Efforts to dry these materials using known types of dryers such as flash, fluid bed, rotary dryers and the like have not been satisfactory both because of their low throughputs and the poor thermal efficiencies, the latter resulting from the necessity of maintaining low inlet temperatures so as to preclude melting the material by contact with hot surfaces. Some measure of success has been achieved using a Lurgi-type drying tower wherein hot air is introduced into the bottom of a hundred foot tower and the solid material into the top whereby relatively long retention times are obtained thus affecting a high level of drying. However, this is done at the expense of low thermal efficiencies combined with the need for utilizing large volumes of hot air which is not only expensive but results in large volumes of noxious gases being discharged into the atmosphere.
It is desirable therefore, to provide a drying technique capable of removing relatively large volumes of mechanically and/or chemically combined water from low melting solids at high thermal efficiencies while minimizing the volume of hot gases used; and to provide relatively inexpensive equipment for effecting these desired ends.